Video is principally a series of still pictures, one shown after another in rapid succession, to give a viewer the illusion of motion. Before it can be transmitted over a communication channel, analog video may need to be converted, or “encoded,” into a digital form. In digital form, the video data are made up of a series of bits called a “bitstream.” When the bitstream arrives at the receiving location, the video data are “decoded,” that is, converted back to a viewable form. Due to bandwidth constraints of communication channels, video data are often “compressed” prior to transmission on a communication channel. Compression may result in a degradation of picture quality at the receiving end.
A compression technique that partially compensates for loss (degradation) of quality involves separating the video data into a “base layer” and one or more “enhancement layers” prior to transmission. The base layer includes a rough version of the video sequence and may be transmitted using comparatively little bandwidth. Each enhancement layer also requires little bandwidth, and one or more enhancement layers may be transmitted at the same time as the base layer. At the receiving end, the base layer may be recombined with the enhancement layers during the decoding process. The enhancement layers provide correction to the base layer, consequently improving the quality of the output video. Transmitting more enhancement layers produces better output video, but requires more bandwidth.
Some compression methods and file formats have been standardized, such as the Motion Picture Experts Group (MPEG) standards of the International Organization for Standardization. For example, one widely-used standard for video compression is denoted as MPEG-2. A compression technique used in MPEG standards encodes changes between individual pictures rather than the pictures themselves. The process results in a substantial saving of bandwidth.